MONDAY, May 31 (HealthDay News) -- Women who take a certain class of antidepressants during pregnancy may increase their risk of having a miscarriage by 68%, Canadian researchers report.

Antidepressant use is common during pregnancy, with up to 3.7% of women taking the drugs during the first trimester. Stopping treatment can lead to a return of depression and other symptoms, and previous studies of the medications' effects on the fetus have been small and had contradictory results.

But the Canadian case-control study on more than 5,000 women found that by controlling for other factors associated with miscarriage, taking antidepressants known as selective serotonin reuptake inhibitors (SSRIs) during pregnancy led to an increased risk of miscarriage.

Up to 20% -- or one woman out of five -- will suffer a miscarriage for various reasons during pregnancy. But the study results suggest that SSRIs as a class increase that risk, according to lead researcher Anick Berard, an associate professor at the University of Montreal.

The results "are highly robust given the large number of users studied," she wrote.

In addition, she said, the study makes clear that the drugs, rather than the mothers' depression and anxiety, are associated with an increased risk for miscarriage.

However, the author of an accompanying editorial noted that the finding is far from definitive.

"This is an association, not a cause," said Adrienne Einarson, assistant director of the Motherisk Program at the Hospital for Sick Children in Toronto. "We still don't know if it's the depression or the drug."

Also, the risk uncovered by the study is a very small one, Einarson added. "Less than twice as many women had miscarriages in the group with antidepressants as those who did not take antidepressants. It's a very small risk indeed, and it's not a reason to stop taking an antidepressant if you need it."

For the study, Berard's team collected data on 5,124 women who had clinically verified miscarriages and compared them with another group of women who had not miscarried.

Of the women who had miscarriages, 5.5% were taking an antidepressant during their pregnancy, the researchers found.

The most commonly used antidepressants were SSRIs. Among these, paroxetine (Paxil) and venlafaxine (Effexor) were associated with a 51% increased risk of miscarriage, Berard said.

The risk of miscarriage also increased with higher daily doses of these drugs. In addition, using a combination of different antidepressants doubled the risk of miscarriage, the researchers noted.

Berard believes that as part of pregnancy planning, women should discuss with their doctor the risks and benefits associated with different types of antidepressants.

"I would certainly advise against using Paxil and Effexor early on in pregnancy," she said. "This doesn't mean women can't use antidepressants; there are others on the market. Planning pregnancy and actually choosing which type of therapy beforehand is an option."

Einarson noted that many women with depression are undertreated.

"My bottom, bottom, bottom line is that if a woman needs to be on an antidepressant, she must continue to take it. This should not be a reason to stop it," Einarson said.

Another expert, Dr. Salih Yasin, associate professor and vice chair of obstetrics and gynecology at the University of Miami Miller School of Medicine, said this study can be useful in guiding doctors in advising patients.

First, one should determine whether the woman should be taking an antidepressant or not, Yasin noted. "There are many people who have depression, but don't need medication," he said.

"With patients who need medications, one has to pick the lowest dose of the ones that have the least association with miscarriage," Yasin said.

The report is published in the May 31 edition of the Canadian Medical Association Journal.